Position a rack in the lower third of the oven and preheat to 425°. In a medium saucepan, add enough salted water to reach a depth of 2 inches; bring to a boil. Add the asparagus and cook until crisp-tender, about 3 minutes. Drain and rinse with cold water, then pat dry with paper towels. Cut the spears crosswise into thirds.
In a medium cast-iron skillet, add the butter. Place the skillet in the oven to melt the butter.
In a medium bowl, microwave the milk on high for 30 seconds. Whisk in the eggs, then the flour, 1/2 teaspoon salt, 1/2 teaspoon pepper and the sugar.
Add the asparagus pieces to the hot cast-iron skillet and pour the batter on top. Sprinkle with half of the cheese and bake until puffed and golden-brown, 18 to 20 minutes. Top with the remaining cheese.